The T-Boom Yagi.
In
late 2008, I started to think about building a new antenna for 1296 MHz. At the time, I was using a single 39-element
DL6WU-style Yagi, constructed to dimensions calculated using the Excel
spreadsheet created by David, VK3AUU, later modified by Peter, VK3PF. It used 4.74 mm aluminium tube for all the
passive elements, mounted through a 12.5 mm box-section aluminium boom. The driven element – a folded dipole - was
made from 3.2 mm copper tubing.
Of
course, the main driver was for more gain, which meant either a longer boom,
with more elements, or multiple antennas in an array. Or both. Naturally, I
chose both…
I
also wanted to use a different element thickness, as the 4.74 mm tubing wasn’t
readily available any more. I’d also
paid attention to people suggesting that using 4.74 (or 6.35) mm tubing at 1296
MHz was akin to using 50 mm tubing for elements at 144 MHz. I really needed to go thinner.
One
person I discussed this with was Gordon, VK3EJ (formally VK2ZAB). Gordon mentioned an idea he had had a few
years earlier, but had never acted upon: that of using “Tee” profile aluminium
as boom material. “Tee” profile looks,
in cross section, like the letter T.
The idea was to turn the material upside down, and mount the elements
through the top of the upright portion of the T (within a millimetre of the
top). With the element in its final
position, the top of the boom would be punched down using a cold chisel,
holding the element firmly in place – full metal-to-metal contact.
The
“Tee” profile chosen for this project has a wall thickness of 3 mm, and the
“upright” and “base-plate” portions of the T measure 25 mm (refer Diagram 1:
Tee Profile). In Australia, it is
available in lengths up to 6.5 m long from Capral, and should be available
elsewhere in shorter (2 m) lengths. The
Capral Product Code for this profile is currently 841506, though it used to be
called “E20193MF605400” in the old Capral “Little Blue Book”.
To
kick things off, I built two 10-element Yagi; one scaled for 1296.200 MHz, and
another scaled for 1090 MHz (for use with my ADS-B receiver). As this was an exercise in a new
construction technique, I used the DL6WU design as the basis for both. This time I used John Drew’s Yagi Calculator
to produce the dimensions (see the link at the bottom if you wish to download a
copy of the software). The resultant
Yagi both appeared to work well. The
1296 MHz 10 element was tested at home with ye olde Revex® W-570 VSWR meter
and, without any adjustment to the elements, the Yagi gave a reading of
1.3:1. The 1090 MHz 10-element Yagi was
swept with the assistance of Alan, VK3XPD; with a little tweaking, a return
loss of 23 dB was achieved. This
tweaking amounted to a little filing of the first director ends, and tilting
the driven element forwards slightly.
Next
I built a 28-element Yagi scaled for 1090 MHz. The 1090 MHz 28-element Yagi is
currently in use at the top of my tower, and is a considerable improvement over
the 16-element co-linear that I was using with my ADS-B receiver. It has a calculated gain of 16.7 dBd.
With
three successes under my belt, I felt it was time to start work on the new 1296
MHz array. This would be four Yagi,
each with 50 elements on 18 wavelength booms.
Work on this was a bit sporadic; fours hours spent cutting the 196
passive elements would leave me wanting to do almost anything else for a
while... But in a burst of work in
January 2011 it all came together, and January 25th saw the array in place and
operational at the top of my tower.
As
a simple exercise in trying to improve signals on 1296 MHz, the new array is a
marked improvement over the single Yagi it replaced. AEP tests on 1296 MHz with Ian VK1BG, in the Canberra suburb of
Hawker, have been taking place for a number of years now. Using the new array, peak signals from Ian
have improved 2 to 3 S-points. Rather
than having three or four "peaks" of 41 to 51 RS reports separated by
nil signal troughs, we usually experience several minutes of 41-51 continuous
signal, with two or three peaks of greater strength; 54 to 56, dependant on the
size of the aircraft.
Before
the array was completed, I wrote a construction article for "Amateur
Radio" magazine. It was published
in the December 2009 edition of the magazine, and with the kind permission and
assistance of the editor it is now also available here in PDF format.
More
photos of the assembly of both the array and the ADSB receive Yagi are viewable
here: http://www.flickr.com/photos/72319077@N00/sets/72157603461538031/
The
VK5DJ Yagi Calculator is available here: http://www.vk5dj.com/yagi.html
Updated
18/5/2011